1. Field of the Invention
The present invention relates to a liquid crystal display device having a reflection electrode and a manufacturing method for the same and, more particularly, to a liquid crystal display device applicable to a transflective type liquid crystal display device which can be used as a reflection type liquid crystal display device in the environment of a bright ambient, and can be used as a transmission type liquid crystal display device by switching on a backlight in the environment of a dark ambient, and a manufacturing method for the same.
2. Description of the Prior Art
A liquid crystal display device is thin and lightweight compared with a CRT (Cathode Ray Tube), having an advantage that it can be driven with a lower voltage with low power consumption. The liquid crystal display device is used for various electronic devices such as a TV set, a notebook type personal computer, a desktop type personal computer, a PDA (Personal Digital Assistance), and a cellular telephone. Especially, an active matrix type liquid crystal display device provided with a TFT (Thin-film transistor) as a switching element for each sub-pixel (hereafter, referred to as a picture element in this invention) shows an excellent display characteristic comparable to the CRT, with its high driving capability. Therefore, the active matrix type liquid crystal display device has been extensively used for the fields where conventionally the CRT is used, such as a desktop type personal computer and a TV set.
Generally, the liquid crystal display device has a structure in which liquid crystal is enclosed between two transparent substrates. A picture element electrode, the TFT and the like are formed for each picture element on one of the two transparent substrates, and color filters disposed opposingly to the picture element electrode and a common electrode that is common to each picture element are formed on the other substrate. Hereafter, the substrate having the picture element electrode and the TFT formed thereon is referred to as a TFT substrate, and the substrate arranged opposingly to the TFT substrate is referred to as a counter substrate. Note that one pixel is formed of three picture elements (sub-pixels) of red (R), green (G) and blue (B).
The liquid crystal display device includes a transmission type liquid crystal display device and a reflection type liquid crystal display device. The transmission type liquid crystal display device displays images by controlling the light quantity of transmitted rays of light for each picture element, while the reflection type liquid crystal display device displays images by controlling the light quantity of reflected rays of light for each picture element. The transmission type liquid crystal display device requires an exclusive light source called a backlight. Meanwhile, in the reflection type liquid crystal display device, an ambient condition of light (natural light or lamplight) is used as a light source. Therefore, the reflection type liquid crystal display device has a merit of consuming much less power compared with the transmission type liquid crystal display device. In addition, the reflection type liquid crystal display device is more excellent in visibility outdoors than the transmission type liquid crystal display device. Hereafter, the picture element electrode of the reflection type liquid crystal display device is also referred to as a reflection electrode.
For example, Japanese Patent Laid-Open No. Hei 08-338993 discloses a reflection type liquid crystal display device in which a TN (twisted nematic) type liquid crystal is used and an alignment film is subjected to rubbing treatment to make the liquid crystal twist align. Moreover, Japanese Patent Laid-Open No. Hei 05-232465 discloses a liquid crystal display device whose reflection electrode is provided with ruggedness by using a photolithography method. In this way, by providing the surface of the reflection electrode with ruggedness, it is avoided that the visibility is greatly changed depending on a position where a panel is observed, by irregular reflection of light.
However, the process for forming ruggedness on the surface of the reflection electrode is complicated in the above-described method. Hereupon, the present inventors provide a method for forming a reflection electrode provided with ruggedness on the surface thereof by using a positive type photoresist (for example, Japanese Patent Laid-Opens No. 2002-221716 and No. 2002-296585). In this method, the step to harden only a surface layer is carried out by subjecting the photoresist to ultraviolet ray irradiation and followed by heat treatment. Fine ruggedness is thus formed on the surface of the resist film. Then, by forming the reflection electrode on the resist film, the reflection electrode having surface ruggedness can be easily formed.
Incidentally, in the reflection type liquid crystal display device, since an ambient condition of light (natural light or lamplight) is used as a light source, the visibility is greatly changed depending on the ambient condition of light. That is, when its neighborhood is bright, the visibility of the reflection type liquid crystal display device is satisfactory. However, when its neighborhood is dark, the visibility thereof is extremely decreased. In order to overcome such a disadvantage, the reflection type liquid crystal display device having a light source (front light unit) on a front panel surface, is proposed. However, the reflection type liquid crystal display device with this structure is formed so that the light reflected by the reflection electrode may be transmitted through the front light unit, where the reflective light is reduced. Therefore, such a reflection type liquid crystal display device poses a problem that contrast of an image is lowered and sufficient visibility is not obtained compared with the reflection type liquid crystal display device without any front light unit.
Japanese Patent Laid-Open No. Hei 07-333598 discloses a liquid crystal display device (hereafter, referred to as a transflective type liquid crystal display device) that can be used as a reflection type liquid crystal display device when its neighborhood is bright, and as a transmission type liquid crystal display device by switching on a backlight when its neighborhood is dark. This is realized by forming a reflection electrode of a metal thin film for semi-transmitting light. However, in this type of transflective type liquid crystal display device, when used as a transmission type liquid crystal display device, light absorption by the metal thin film is increased. Therefore, the utilization efficiency of light is bad, involving a problem that satisfactory visibility cannot be obtained unless a backlight having large luminance is used. Al (aluminum) film having a thickness of about 30 nm is used as the metal thin film for semi-transmitting light. However, in the case of a large-sized liquid crystal display device, it is extremely difficult to form an Al thin film having a uniform thickness over the entire surface of a panel.
Japanese Patent Laid-Open No. Hei 11-281972 discloses a transflective type liquid crystal display device in which the central part of a reflection electrode is opened to form a transmission region through which light is transmitted, and in the transmission region, a transparent electrode such as an ITO (Indium-Tin Oxide) is formed.
FIG. 1 is a schematic diagram showing an example of the TFT substrate of a conventional transflective type liquid crystal display device with the above structure.
On the TFT substrate, a plurality of gate bus lines 71 disposed so as to be parallel to each other, and a plurality of data bus lines 72 so as to be orthogonal to the gate bus lines 71, are formed. In the vicinity of each area where the gate bus line 71 and the data bus line 72 intersect with each other, a TFT 73 is formed. Moreover, in each rectangular region partitioned by the gate bus lines 71 and the data bus lines 72, a reflection electrode 74 made of a metal film for reflecting light such as Al (aluminum) is formed. In the central part of the reflection electrode 74, an opening part 74a for transmitting light is formed, and in the opening part 74a, a transparent electrode 75 made of a transparent electric conductor such as ITO is formed.
The gate bus lines 71, the data bus lines 72, and the TFTs 73 are covered with an insulating flattening film; the reflection electrodes 74 are formed on the flattening film; and the transparent electrodes 75 are formed under the flattening film. When direct contact of the Al constituting the reflection electrodes 74 and the ITO constituting the transparent electrodes 75 occurs, corrosion is caused due to a battery effect. For this reason, the reflection electrodes 74 and the transparent electrodes 75 are electrically connected via a barrier metal such as Ti (titanium).
In the liquid crystal display device with the above structure, scanning signals are sequentially supplied to a plurality of the gate bus lines 71, and display signals are supplied to each of the data bus lines 72 when displaying an image. Then, the TFTs 73 connected to the gate bus lines 71 supplied with the scanning signals become in ON states, and the display signals are written in the reflection electrodes 74 and the transparent electrodes 75 via the TFTs 73, whereby the orientation of liquid crystal molecules between the reflection electrodes 74 and the counter substrate, as well as the transparent electrodes 75 and the counter substrate, are changed. Consequently, the light quantity of the reflective light or the transmitted light is also changed. By controlling the light quantity of the reflective light or the transmitted light for each picture element, a desired image is displayed on the liquid crystal display device.
According to the transflective type liquid crystal display device, comparatively satisfactory visibility is secured in any case of using it as the reflection type display device or as the transmission type display device.
However, in the transflective type liquid crystal display device disclosed in Japanese Patent Laid-Open No. Hei 11-281972, the transparent electrodes made of ITO and the barrier metal are required to be formed in addition to the reflection electrodes made of Al. Accordingly, there arises a problem that many processes are required, involving an increase in product costs.
Furthermore, in this transflective type liquid crystal display device, if the transmission region is enlarged, the reflection region is reduced. Transmission and reflection characteristics are thus defined by a trade-off relation. In a liquid crystal display device with high resolution, the area of one picture element is small. Therefore, it is difficult to obtain a satisfactory liquid crystal display device in reflection characteristics as well as in transmission characteristics.
Further, in the reflection region, incident light is transmitted through CF (color filter) layers two times and emitted to a display screen side. Meanwhile, in the transmission region, incident light is transmitted through the CF layers only once and emitted to the display screen side. For this reason, chromaticity irregularity is generated between the cases of using the transflective type liquid crystal display device as a reflection type liquid crystal display device (hereafter, referred to as a reflection mode) and as a transmission type liquid crystal display device (hereafter, referred to as a transmission mode).
When the color purity of the CF layers is adjusted so that a bright display can be obtained when displaying in the reflection mode, the color purity in the transmission mode is deteriorated, resulting in a display in light colors. Conversely, when the color purity of the CF layers is adjusted so that satisfactory color rang can be obtained when displaying in the transmission mode, reflected light is lowered when displaying in the reflection mode, resulting in an extremely dark display.
In order to overcome the above-described problems, a structure of the liquid crystal display device is conventionally known, in which the color purity of the CF layers is made to be different between the reflection regions and the transmission regions (for example, Japanese Patent Laid-Open No. Hei 11-2811). In this structure, for example, the CF layers are not formed in the reflection regions, but formed in the transmission regions only. Accordingly, when displaying in the transmission mode, a display with high color purity can be obtained, and when displaying in the reflection mode, a display with high luminance, in achromatic colors though, can be obtained. However, this structure involves a problem that display quality is greatly changed when the reflection mode and the transmission mode are switched with each other. Moreover, in the reflection mode, a full-color display cannot be provided, involving a problem that a transmittable information quantity for users via the display screen is reduced and good display quality cannot be obtained.
In order to overcome the above-described problems, another structure of the liquid crystal display device is conventionally known, in which color purity is made to be different between the reflection regions and the transmission regions (for example, Japanese Patent Laid-Opens No. Hei 11-305248 and No. 2001-166289). However, in this structure, while the chromaticity irregularity between the transmission mode and the reflection mode can be reduced, reflection characteristics and transmission characteristics are still defined by a trade-off relation. Therefore, it is difficult to improve both of the reflection characteristics and the transmission characteristics, raising a problem that utilization efficiency of light is degraded.
Also, there is provided a reflection type liquid crystal display device in which the color purity of the CF layers is made to be different for each picture element region (for example, Japanese Patent Laid-Open No. Hei 10-307205). In this structure, a display is produced using picture elements of six colors in total including three colors of red (R), green (G) and blue (B) with the addition of complementary colors thereof of cyan (C), magenta (M) and yellow (Y), thereby enlarging the range for color reproduction. However, an increase in a drive circuit leads to an increase in the manufacturing costs, and therefore the liquid crystal display device provided with the picture elements of six colors may not be practical. In addition, the above structure is not applicable to the transflective type liquid crystal display device.
The transflective type liquid crystal display device that improves the transmission characteristics without decreasing the reflection characteristics, is proposed in Japanese Patent Laid-Open No. 2003-202594 filed by the present applicant. This transflective type liquid crystal display device will be explained with reference to FIGS. 2 to 5. FIG. 2 shows a structure of the TFT substrates of the transflective type liquid crystal display device, and FIG. 3 shows a sectional structure of the transflective type liquid crystal device taken along the line I—I of FIG. 2. As shown in FIGS. 2 and 3, reflection electrodes 110 are formed so as to cover gate bus lines 104, data bus lines 106, and TFTs 108. The regions where the reflection electrodes 110 are formed serve as reflection regions R and R′. The regions surrounding the reflection electrodes 110 serve as transmission regions T and T′. The liquid crystal in the transmission regions T and T′ are driven similarly to the liquid crystal in the reflection regions R and R′ by an oblique electric field between the reflection electrodes 110 and a common electrode 130.
In this structure, the regions used neither as the reflection regions nor as the transmission regions in the conventional liquid crystal display device, are used as the transmission regions T and T′. Moreover, the areas of the gate bus lines 104, the data bus lines 106, and the TFTs 108 which are exposed in the transmission regions T and T′, are decreased to a large extent. Therefore, the areas of the transmission regions T and T′ can be enlarged without decreasing the areas of the reflection regions R and R′. Accordingly, the transmission characteristics can be improved without decreasing the reflection characteristics, and good display characteristics can be obtained in both of the reflection mode and the transmission mode.
FIG. 4 shows another structure of the transflective type liquid crystal display device. As shown in FIG. 4, the reflection electrodes 110 are formed in regions surrounded by the gate bus lines 104 and the data bus lines 106. The regions where the reflection electrodes 110 are formed serve as reflection regions. The reflection electrodes 110 have opening parts 150a to 150e formed therein. The opening parts 150a to 150e are opened to be formed into various shapes such as a slit-like shape and a circular or polygonal hole-like shape. The regions where the opening parts 150a to 150e are formed serve as transmission regions.
FIG. 5 shows further another structure of the transflective type liquid crystal display device. As shown in FIG. 5, the reflection electrodes 110 are formed so as to cover the gate bus lines 104, data bus lines 106, and the TFTs 108. The regions where the reflection electrodes 110 are formed serve as reflection regions. The reflection electrodes 110 have opening parts 150f to 150k formed therein which are opened to be formed into a slit-like shape, a circular or polygonal hole-like shape and the like. The regions where the opening parts 150f to 150k are formed and the regions between the adjacent reflection electrodes 110 serve as transmission regions.
In the structures shown in FIGS. 4 and 5, transparent electrodes such as ITO are not formed in the transmission regions. Therefore, the transparent electrodes and a barrier metal layer are not required to be formed therein. Moreover, for example, by forming the opening parts 150a to 150k into a shape enabling the orientation control of a liquid crystal having negative dielectric anisotropy, rubbing treatment for an oriented film can be eliminated. Accordingly, a manufacturing process of the liquid crystal display device is simplified and manufacturing costs are reduced.
As described above, according to the transflective type liquid crystal display devices as shown in FIGS. 2 to 5, the transmission characteristics can be enhanced without decreasing the reflection characteristics. At the same time, the manufacturing process can be simplified and the manufacturing costs can be reduced. However, this transflective type liquid crystal display device still has such problems as will be described below. FIG. 6 shows a schematic sectional structure of three picture elements of the liquid crystal display device taken along the line II—II of FIG. 4. As shown in FIG. 6, a light beam t of the transmission light emitted from a backlight unit (not shown) to be emitted to a display screen side, and a light beam r of the reflection light incident from the display screen side and reflected by the reflection electrode 110 to be emitted to the display screen side, pass along different light paths. That is, the light beam t of the transmission light is transmitted through the CF layer R only once. On the other hand, the light beam r of the reflection light is transmitted through the CF layer R twice. Therefore, there arises a problem that color purity is made to be different between the transmission mode display and the reflection mode display, thereby degrading display quality.
Moreover, in order to obviate the occurrence of difference in color purity between the transmission mode display and the reflection mode display, it is conventionally known that the film thickness of the CF layers in the transmission regions is made twice as thick as CF layers in the reflection regions. However, in this structure, alignment margins for tolerating alignment deviation generated when aligning a TFT substrate 102 having the reflection electrodes 110 formed thereon with a counter substrate 114 having the CF layers formed thereon, cannot be secured. For this reason, there arises a problem that when the alignment deviation is generated, color purity is made to be different between the transmission mode display and the reflection mode display, thereby degrading the display quality.